Pycnogonum stearnsi Ives, 1892

Common name(s): Stearns' sea spider

Phylum Arthropoda
 Subphylum Chelicerata
  Class Pycnogonida
   Order Pantopoda
    Family Pycnogonidae
Pycnogonum stearnsi from Little Corona del Mar, CA.  Total leg span about 1 cm.  Animal was in a tidepool next to the base of an Anthopleura sola anemone.  The head with its large proboscis is facing left (there is a bubble at the tip of the proboscis)
(Photo by: Dave Cowles, November 2006)
Description:  This animal is NOT a crustacean.  As a chelicerate, most of the species in this group have chelicerae instead of jaws.  Its legs are relatively thick for a pycnogonid, and the individual legs are not conspicuously longer than the combined length of the proboscis and trunk.  The trunk is distinctly segmented.  The overall outline of the animal is oval or elliptical.  It has no chelicerae or pedipalps and no eyes or conspicuous spiny projections.  The color is a solid white or ivory to pink or tan.  If any dorsal tubercles are present they are not taller than their diameter.  The legs end in claws.

How to Distinguish from Similar Species: Pycnogonum rickettsi is similar in shape and also lacks chelicerae and pedipalps, but its body is usually light brown with dark lines or patches and it has dorsal tubercles which are taller than their diameter.

Geographical Range:  British Columbia to southern California; Japan.  Most common in central California.

Depth Range: Mid to low intertidal

Habitat:  Typically found under rocks, in empty barnacle shells, or on large anemones, especially on Anthopleura xanthogrammica.  May also feed or be found on the anemone Metridium senile, on hydroids such as Obelia and Aglaophenia, on ascidians, or in surfgrass.

Biology/Natural History:  Males have a pair of accessory (ovigerous) legs for carrying eggs.  Females do not have these legs.  This species seems to preferentially live on the giant green anemone Anthopleura xanthogrammica.  Several individuals may occur on the same anemone (see below).  It is one of the largest sea spiders to be found along our coast.  The species feeds by jamming its proboscis into the anemone's tissues and sucking fluids.  The proboscis has a wide aperture, probably to allow sucking up some particles in its food.  Studies of closely related species have suggested that the animal has no heart (it probably pumps its blood by peristalsis).

Family Pycnogonidae is one pycnogonid family in which females lack ovigerous legs entirely.  This is true for some other families as well, while in others the females have reduced ovigerous legs.  Females release eggs from gonopores which are present on several legs near the base.  The male, which is standing over or under her, fertilizes the eggs then gathers them up and sticks them to his ovigerous legs, where he cares for them.  The eggs hatch as small protonymphon larvae which can swim.  In a related closely related species to this one (P. litorale), the larva loses all three pairs of its larval legs and its proboscis at the fifth molt, then produces the adult proboscis and legs during later molts.

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Dichotomous Keys:
  Kozloff 1987, 1996
  Smith and Carlton, 1975

General References:
  Brusca and Brusca, 1978
  Johnson and Snook, 1955
  McConnaughey and McConnaughey, 1985
  Morris et al., 1980
  Ricketts et al., 1985

Scientific Articles:
Behrens, W., 1984.  Larventwicklung und Metamorphose von Pycnogonum litorale (Chelicerata, Pantopoda).  Zoomorphologie 104: 266-279

Tomaschko, K.H., E. Wilhelm, and D. Buckermann, 1997.  Growth and reproduction of Pycnogonum litorale (Pycnogonida) under laboratory conditions.  Marine Biology 129: 595-600

Wilhelm, E., D. Buckermann, and K.H. Tomaschko, 1997.  Life cycle and population dynamics of Pycnogonum litorale (Pycnogonida) in a natural habitat.  Marine Biology 129: 601-606

Web sites:

General Notes and Observations:  Locations, abundances, unusual behaviors:

This pair of individuals, which includes the individual shown at the top of the page, were both on the same anenome.
Though I did not check thoroughly, I did not see ovigerous legs on either individual suggesting they are both female.

These two individuals, a female on the left and a male on the right, were found on the same anemone near Cape Flattery in summer 2014.  This is the dorsal view.  The undersides (ventral views) are shown below to help distinguish between male and female in this species.
Female underside Male underside
Underside of female.  Note that there are no ovigerous legs arising near the base of the proboscis Underside of male.  Note that a short pair (only 9 segments) of ovigerous legs arise near the base of the proboscis.

This view shows the underside of the proboscis.

Authors and Editors of Page:
Dave Cowles (2006):  Created original page